Japanese/English

General Review

Development and Prospective of Ocean Thermal Energy Conversion and Spray Flash Evaporator Desalination

Haruo UEHARA* and Tsutomu NAKAOKA**

^*Faculty of Science and Engineering, Saga University, 1 Honjyo-machi Saga City, Saga, Japan, 840-8502
Phone +81-952-28-8607
FAX +81-952-28-8595

^** Tsutomu NAKAOKA, Department of Ocean Mechanical Engineering, National Fisheries University, 2-7-1 Nagatahon-machi Simonoseki City, Yamaguchi, Japan, 759-6595
Phone +81-832-86-7381
FAX +81-832-86-7381

1. Introduction

  Energy, water, air and food are the most important things for human surviving. At present, the energy resources are mainly from fossil fuels, such as oil, coal, gas, etc. Because of the emission of the gas, which causes the green effect, the utility of fossil fuels results a big problem in the worldwide. According to the world convention on the decrease of the gas emission, it is necessary to develop the fossil furl replacements and the technology on energy saving. Now various kinds of fossil fuel replacements are being developed, including solar energy, geothermal energy, wave energy, wind energy, biomass, etc. However, the development of those kinds of energy is still far from enough for the current energy demands. In this paper, we introduce the development of Ocean Thermal Energy Conversion (OTEC), which contribute a huge energy resource.
  On the one hand, the demand for water increases along with the rapid industry development, population increase and the improvement of the living standard; on the other hand, it is also difficult to find the water resources. Furthermore, the distribution of the rainfall is very different in the different area owing to the recent abnormal weather, and in some regions, the freshwater is mixed with seawater owing to the warming of earth weather and the ascending of the sea level. Therefore, water economizing and purification of river are being implemented in the regions where the water supply is less than the water demand. More unluckily, some countries or regions are lack for not only the energy but also the water resource. Therefore, for those countries and regions, both the energy replacement system and seawater desalination are necessary.
  The authors have developed a way to solve those two problems simultaneously, in which the energy is obtained using Ocean Thermal Energy Conversion (OTEC) and the water resource is obtained by Spray Flash Evaporator Desalination (SFED)[18][31][33] . This paper introduces the development and prospective of ocean thermal energy conversion and spray flash evaporator desalination, and the developed integrated hybrid OTEC cycle.

2. Ocean thermal energy and possible setting regions

  The ocean receives the solar energy and stores it in thermal energy. The energy from sunlight radiation is about 55.1x10¹² kW/s, and 2% of that energy, e.g. 1.1x10¹² kW, is useful, which is about 100 times of the energy consumed in the world in 2000 [1].
  Woff et al. [2] have investigated the temperature distribution in the world, in which the temperature differences between the surface and 1000 m depth of the seawater is illustrated in Fig. 1. From the figure, the regions, which the temperature difference is more than 16-centigrade degree, are between north 40-degree latitude and south 40-degree latitude. As the minimal temperature difference for an economic OTEC plant is 13-centigrade degree, the OTEC plant can be constructed in the countries or islands between north 40-degree latitude and south 40-degree latitude. Further, since the seawater can be desalinated under SFED system if the temperature difference is more than 5-centigrade degree, the warm and cold seawater at the outlet in OTEC plant can be utilized.

Fig.1 Temperature difference between sea water at ocean's surface and sea water at the depth of the 1000 m

3. Development of ocean thermal energy conversion

 A brief history of the development of OTEC is shown in Table 1.

Table 1 Major stages in the development of OTEC

  OTEC history dates back to 1881, when a French scientist J. D'Arsonval had come up with the idea of OTEC power plant [3]. Since then, G. Claude had ventured many times to commercialize the OTEC power generation from 1926, but all of the attempts were in vein. However, the by Anderson had achieved a great success in 1964 after a temporal blank in OTEC study [5-7]. 
  The very starting of OTEC study was 1974, when the ERDA project in the USA and the Sunshine project in Japan were permitted. In Japan, the main OTEC study is approached in Saga University and the Institute of Electronic Technology of Industry Technology Academy, The main results obtained by the authors are listed as follows. 
  Uehara et al. [8] made an experimental test on the constructed OTEC plant "Shiranui 3". At the beginning, Freon 114 was used as the working fluid, and an electrical output of 1kW was achieved. Further, the continuous running data of OTEC plant and the performance of the evaporator and condenser using shell and tube type were evaluated. Uehara et al. [9] made an experimental test on the constructed OTEC plant using Freon 22 as working fluid and plate type heat exchanger. The performance of plate type heat exchanger was analyzed, and the results of 25MW OTEC plant using Freon 22 were compared to the previous test [8]. 
  Uehara et al. [10] calculated the optimal design of both heat exchanger and turbine, for different working fluids, ammonia and Freon 22. 
  Uehara et al. [11] reported the experimental results of OTEC plant on Japanese sea. The plate type heat exchanger was used, and the cold-water upwelling pipe was made of steel. Bending moment of cold-water upwelling pipe was 3.52x10³ Nm at a depth of 30m and 1.24x10³ Nm at a depth of 102m. The turbine rpm was 1100. 
  Uehara et al. [12] reported the construction of a 50 kW OTEC in Imari. The continuous running data of the temperature, mass flow and heat transfer coefficient of the heat exchanger under the actual seawater were measured. 
  Uehara et al. [13] developed a computer program for obtaining an optimal output under a minimal heat transfer area of a 100MW OTEC plant, in which the plate type heat exchanger was used with ammonia as the working fluid. Further, a comparison result with double fluted type heat exchanger was reported, in which a net electricity output of 66 - 70% was shown [14]. Uehara et al. has also verified that the ammonia the best working fluid compared to Freon 22 under the same condition of seawater, where it was found that the net output using Freon 22 is 6% less than that of the ammonia [15]. 
  Uehara et al. [16] optimized a 3000kW OTEC plant for the isolated islands. 
  Uehara et al. [17] investigated eight different areas in Philippine Sea, and completed the conceptual design of the OTEC plant in those areas. 
  Uehara et al. [18] investigated the ocean condition around Okinoerabu, and analyzed the performance of the integrated hybrid cycle combined with OTEC and seawater desalination equipments with 10MW electrical output, the overall heat transfer area was 21.5 m²/ kW for a positive net output under the condition that the temperature difference between the inlets of warm and cold seawater was 20°C. 
  In the above researches, the Rankine cycle, which uses pure substances like ammonia as the working fluid, was the main object. In 1981, Dr. Kalina introduced his new invention of Kalina cycle, in which ammonia / water mixture was used as the working fluid. 
  Uehara et al. [19] verified the effectiveness of utilizing Kalina cycle for an OTEC plant, where they analyzed the thermal efficiency of cycle, the concentration of ammonia / water mixture, the heat transfer performance of the regenerator, the effect of the temperature and pressure at the inlet of the turbine. As a result, under the condition that the temperatures of the warm and cold seawater were 28 and 5 °C, respectively, the thermal efficiency of cycle can reach 5%. 
  Uehara et al. [20] developed a new cycle (Uehara Cycle) with absorption and extraction processes, and demonstrated the cycle performance. In this cycle, ammonia / water mixture was used as the working fluid. Compared with Kalina cycle, the thermal efficiency of Uehara cycle is about 1 - 2 % higher in theoretical consideration. After operating the cycle with 9 kW, experimental equipments, there was an agreement between the experimental results and theoretical calculation. Since 1997, Saga University has been implementing a joint development of a 1 MW practical OTEC plant with National Institute of Ocean Technology, India (NIOT). In 2000, the construction of this practical OTEC plant was stood in Indian Sea. After finishing the construction, Uehara Cycle will be implemented. After the evaluating performance of the 1 MW OTEC, a 25 - 50 MW commercial OTEC plant will be constructed. 

Fig.2 OTEC system using a cycle with absorption and extraction processes
(Uehara cycle)

  In 1977, a 750W (ETL-OTEC-II) experimental electrical plant was implemented in the Electro technical Laboratory (Japan) [21]. Using Freon 114 as the working fluid, the necessary data for the design of a commercial OTEC plant was obtained. In 1989, an OTEC plant was constructed and experiments were carried out in the sea of Toyama bay.
  In 1981, Tokyo Electric Company  et al. constructed and  studied a plant with electricity output of 100kW   in Republic of Nauru .[22]  This plant employed with Freon 22 as the working fluid and obtained 120kW in maximum output power.
  In 1982, Tokunoshima plant was constructed by Kyushu Electric Company [23]. The closed loop cycle was used and electricity was generated utilizing the waste heat of diesel engine where the electricity output was 50kW. 
  In USA studied the OTEC cycle, and in 1979, the Mini-OTEC cycle with output of 50kW was built in Kiaholepoint  of Hawaii [24]. The ammonia was used as the working fluid, and the heat exchanger was plate type. 
  An open loop experimental plant [25] was constructed at the Kona beach offshore in Hawaii in 1993. The temperatures of warm and cold seawater were 27.5 and 6.1°C. The total power capacity was 255kW where the consumed power was 152kW and the net power was 103kW.

4. Study on the spray flash evaporation

  Miyatake et al. [26] proposed the spray flash evaporation method that can evaporate the seawater under relatively low temperature and efficiency. In this evaporation method, the temperature of liquid was released directly to the container that was decompressed under saturation pressure and temperature, via the nozzle. In MSF evaporator, it has evaporation limitation of static pressure rise. This method does not have these phenomena. By using this method can induce the evaporation quickly and completely. 
  Miyatake et al. [27]-[28] studied the influence of degree of superheat, flow rate, diameter of nozzle and liquid temperature on this spray flash evaporation. Further, Miyatake et al. [29] studied experimentally the spray flash evaporation providing the artificial nucleus *** for the superheated liquid. This study indicated that it could promote remarkable evaporation and reduce the non-equilibrium temperature difference at high temperature and degree of superheat. He also had investigated the cause of high-performance-SFED. 
  Bharathan - Penney [30] studied experimentally the evaporation performance of a falling turbulent planar-shattered jets and a vertical spout evaporator. 
  Uehara et al. [31] optimized the hybrid cycle, which combines OTEC with seawater desalination method in order to utilize the ocean thermal energy effectively. Then, they employed with the SFED as a seawater desalination method and developed the optimization method for combining OTEC cycle with SFED. Further, they [32] studied experimentally to develop the spray flush device in order to improve the performance of hybrid cycle. The study indicated that it is possible to evaporate the seawater well using the spray flush evaporation process if the liquid temperature is 30°C. 
  Uehara et al. [33] implemented the performance analysis for Integrated-Hybrid OTEC (I-H OTEC) cycle, which consist of OTEC and SFED, and it was also compared with Joint-Hybrid OTEC (J-H OTEC) They reported that the I-H OTEC cycle can obtain 33 - 80% higher desalination ratio than the J-H OTEC. 
  Uehara et al. [34] measured the liquid temperature in the flush desalination experiment, which is employed with working fluid (tap water) and different three types of nozzles that is four holes, cylinder and oval. It was found that the number of nozzles depends on the degree of superheat and liquid flow rate. They also reported [35-36] that the influence of nozzles numbers on the flush desalination experiment employed with one to six nozzles whose diameter d is 10.0 mm and length l is 81.3mm. Under these conditions, the number of nozzles also depends on the degree of superheat and liquid flow rate. 
  Miyatake et al. [37] investigated experimentally the spray flush evaporation and the efficiency of evaporation rate in order to apply in the seawater desalination process and collection of waste heat.
 Uehara et al. [38] investigated about the I-H OTEC cycle using SFED at the Okinoerabu sea area.
 Uehara et al. [39] studied about the SFED by changing nozzle materials, diameter, average flow rate into the nozzle and flow out rate in order to use it into the I-H OTEC cycle and to obtain the general equations that can use the nozzle flow out temperature under 30 °C. 
  Nakaoka et al. [40] studied optimization method for SFED using above results. Recently, the SFED is watched with keen for small-scale desalination from all directions and investigated for putting into practical one.

5. Study on hybrid system for effective utilization

  For the effective application of the technologies on the energy economics and unutilized energy, the detail discussion on the overall system design is required. Therefore, a hybrid system by combination of some new systems is considered. There are a heat pump system using geothermal energy, discharged warm water from factory, discharged gas, ocean thermal energy by combining OTEC and heat pump system, OTEC, seawater desalination equipments and solar pump, a joined hybrid system by combining OTEC and seawater desalination equipments, and an integrated hybrid system by combining OTEC and water desalination equipments.
  Rabas et al. [41] are engaged in the study on the concept design of the J-H OTEC cycle for both power generation and flesh water production. Further, Rabas and Panchel [42] reported the performance analysis of a system for flesh water production using ocean thermal energy.
  Rabas et al. [43] studied the optimization of a no-gas system for the J-H plant. Further, Rabsa et al. [44] showed the OTEC plant design and cost analysis for both power generation and flesh water production.
  Authors have developed an Integrated-Hybrid OTEC cycle (I-H cycle). The results are shown in the follows.
  The principal diagram of the integrated hybrid OTEC cycle combining a closed OTEC cycle and spray flash evaporator desalination equipments is illustrated in Fig. 3.

Fig.3 Schematic diagram of the I-H OTEC cycle

  The T-S diagram of the I-H OTEC cycle is shown in Fig. 4. The working fluid is transported to evaporator by loop pump, where it is heated by surface warm seawater, and evaporated to be the vapor. The electric power is generated by the whirling of the turbine and electricity generator driven by the vapor passing through. The vapor from turbine is cooled to be water again by the deep seawater in the condenser. Meanwhile, the warm seawater after the heat exchange in the evaporator sprays out from nozzle, and is diffused by vacuum pump. In the flesh evaporation room, the warm seawater is evaporated by spray flesh evaporation. The generated vapor is sent to the condenser for water production, and cooled into fresh water by cold seawater from OTEC condenser with heat exchanged.

Fig.4 T-s diagram of the I-H OTEC cycle

5.1 Evaluation function

  Because of the small temperature difference of OTEC, the heat transfer area is larger than that of other electricity generation method. In I-H cycle, the condenser for water production must be set so that the heat transfer area is increased compared with that in OTEC. Further, a great amount of water is used in I-H cycle similar to the OTEC system. For the performance advance of the spray flesh evaporation, the vacuum pump is required in the seawater desalination equipments so that the consumed pump power is largely increased. Therefore, similar to the case of OTEC system, the optimal system of I-H cycle is evaluated using evaluation function γ for minimizing the heat transfer area and pump power,

so that the I-H cycle is optimized if γ is minimal.

5.2 Condition and calculation method

  From Eq. (1), there are five independent design factor, design condition factor C, shape variable G, state variable S, working variable D, pipe variable P, which determine the evaluation function γ.

  Then, the evaluation function γ can be written as

γ  = f (C, G, S, D, P)

(2)

where C, G, S, D and P are defined as

  From Eq. (1) to (7), ΔX_E, ΔX_C, ΔX_fc, ΔL_E, ΔL_C,ΔL_fc, ΔY_WS, ΔY_CS, (ΔY_CS)_DC, (ΔY_WF)_E, (ΔY_WF)_C, (ΔY_WF)_DC, δ_E, δ_C, δ_DC, T_fc, η_PWS, η_PCS, η_PWF, η_PV, η_T, η_g,　d_WS, d_CS, d_DC, l_WS, l_CS, l_DC, l_fc, w_fc, d_n and l_n are input data, by setting the design factor C, the evaluation function γ can be written as

  By changing the values of the five independent variables, the minimal value of γ is obtained by using fast convergent algorithm introduced in OTEC system design. At first, γ is calculated at an arbitrary condition. Then, fixing other variables, only modifying one variable (for example, T_E) γ₁ is calculated. For the gradient (γ₁ -γ)/ΔT_E of T_E, assigns a new initial value of T_Ｅ. For an arbitrary step interval ε, turns to the next loop. The other variables are similar. Then, the minimal value of γ can be obtained.

5.3 Minimal evaluation function

  The relationship between (T_WSI - T_CSI) and γ_min is shown in Fig. 5. For any T_WSI,T_CSI, the γ_min is almost determined by (T_WSI - T_CSI) so that it can be approximated by

  In Fig. 5, the broken line shows the case of J-H cycle [31], and

  From Fig. 5, when the temperature difference (T_WSI - T_CSI) at the inlet of warm and cold seawater is less than 25K, the γ_min of the I-H OTEC cycle is less than that of the J-H OTEC cycle, whereas it tend to the same if the temperature difference is larger than 25K. In the case that the inlet warm seawater is T_WSI = 28°C and inlet cold water is T_CSI = 5°C, γ_min of the I-H OTEC cycle is 12.22 m²/kW about 12% less than that of J-H OTEC cycle (13.88 m²/kW).

Fig.5 Minimum value of the objective function

5.4 Net power and pump power

  The relationships between temperature difference (T_WSI - T_CSI) at the inlet of warm and cold seawater and the net power P_N, pump power for warm and cold seawater P_WS, P_CS, pump power for the working fluid P_WF, vacuum pump power P_V are illustrated in Fig. 6, where the broken lines correspond to the P_N, P_WS, P_CS, P_V of the J-H OTEC cycle. From Fig. 6, the net power of the I-H OTEC cycle increases along with a higher temperature difference, owing to the decrease of P_WS, P_CS, P_V with a higher temperature difference. 
  From Fig. 6, P_N, P_WS, P_CS, P_V can be approximated as,

  From Fig. 6, the pump power of the I-H OTEC cycle is less than that of J-H OTEC cycle. In the case that the inlet warm seawater is T_WSI = 28°C and inlet cold water is T_CSI = 5°C, the pump power for the warm seawater of the I-H OTEC cycle is about 14% less than that of J-H OTEC cycle. 
  In the case that the inlet warm seawater is T_WSI=28°C and inlet cold water is T_CSI = 5°C, the pump power for the cold seawater of the I-H OTEC cycle is about 17% less than that of J-H OTEC cycle. 
  The vacuum pump power of I-H OTEC cycle is almost the same as that of the J-H OTEC cycle. 
  The pump power for the working fluid of I-H OTEC cycle is about 0.20 - 0.23 MW, almost the same as that of the J-H OTEC cycle.

Fig.6 Net power and pumping power

5.5 Water production

  The relationship between temperature difference (T_WSI - T_CSI) at the inlet of warm and cold seawater and the water production m_DW is illustrated in Fig. 6, where the broken lines correspond to that of the J-H OTEC cycle. 
  The water production of the I-H OTEC cycle decreases along with temperature difference (T_WSI - T_CSI) at the inlet of warm and cold seawater, because the mass flow in the evaporator of OTEC and the mass flow into the flash evaporation room are lees with the higher temperature difference. In the case that the inlet warm seawater is T_WSI = 28°C and inlet cold water is T_CSI = 5°C, the water production of the I-H OTEC cycle is about 35% larger than that of J-H OTEC cycle, because the liquid temperature largely deceases in the flash evaporator room.

Fig.7 Desalination rate

5.6 Desalination rate

  The relationship between temperature difference (T_WSI - T_CSI) at the inlet of warm and cold seawater and the desalination rate is illustrated in Fig. 6, where desalination rate is calculated by (m_DW/m_WS)×100%. (Generally, the water production rate using flash evaporation is the rate of water production and heated vapor (larger than 1.0).) 
  For the J-H OTEC cycle, the desalination rate decreases with a higher temperature difference. For the I-H OTEC cycle, because of the increment of overheat in the flash evaporation room, the rate of water production rate increases with a higher temperature difference. In the case that the inlet temperature difference between warm and cold seawater is about 19-27K, the rate of water production of the I-H cycle is about 0.80-1.00%. 
  For the J-H OTEC cycle, the rate of water production is about 0.60-0.55%. It demonstrates that the rate of water production of the I-H OTEC cycle is about 33-80% larger than that of the J-H cycle.

Fig.8 Desalination ratio

5.7 Heat transfer area

  The relationships between the inlet temperature difference of warm and cold seawater and the overall heat transfer area A_T, heat transfer area of evaporator A_E, heat transfer area of condenser A_C, heat transfer area of condenser for water production are shown in Fig. 9, there the broken lines correspond to the A_T,A_E,A_C of the J-H OTEC cycle. From Fig. 9, A_T, A_E, A_C, A_DC are approximated by

  From Fig. 9, In the case that the inlet warm seawater is T_WSI = 28°C and inlet cold water is T_CSI = 5°C, the heat transfer area of the evaporator in the I-H OTEC cycle is about 8% larger than that of J-H OTEC cycle.
  The heat transfer area of the condenser in the I-H OTEC cycle is about 33% less than that of J-H OTEC cycle.
  The overall heat transfer area of the I-H OTEC cycle is about 8% less than that of J-H OTEC cycle.

Fig.9 Heat transfer area

6. Conclusion

  The development of the effective utilization of the energy replacement based on the development history of OTEC, SFED, has been described. For the effective utilization of economic energy and unutilized energy, the considerations on the overall system for OTEC and various kinds of system are required. Therefore, a hybrid system has been developed. Further, in order to protect the earth environment, new mixtures for the system working fluid is being approached.

7. Nomenclature

Subscript

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